1. Field of the Invention
The present invention relates to a digital data recording and reproducing system for receiving digital broadcasting that multiplexes and transmits a plurality of programs containing video, audio, data and so forth passing through data compression, for recording the received compressed programs as digital data, and for reproducing the recorded digital data.
To be more specific, as a medium for straightforwardly recording and reproducing digital broadcasts that use compressed video, audio, data and so forth passing through data compression according to MPEG (Moving Picture Experts Group)-2, there are digital VTRs, DVDs (Digital Video Disks) and hard disks for simply recording the digital broadcasts without decoding them according to the MPEG-2.
2. Description of Related Art
FIG. 6 is a block diagram showing a configuration of a conventional digital data recording and reproducing system described on page 31 of the Journal of Nikkei Electronics of 1999.7.12. In this figure, the reference numeral 10 designates a set-top box as a home use terminal, 11 designates a tuner, 12 designates an MPEG-2 decoder, 13 designates an IEEE1394 interface, 20 designates a digital VTR, 21 designates an IEEE1394 interface, 22 designates a recorder, and 23 designates a tape.
Next, the operation of the conventional system will be described.
The material to be recorded and reproduced in this example is CS (Communication Satellite) digital broadcasting. The broadcast waves of the CS digital broadcasting are supplied to the set-top box 10, and the tuner 11 selects the carrier of a desired frequency. In the CS digital broadcasting, one frequency carrier includes a plurality of programs multiplexed thereon.
In the examples of FIG. 6, the carrier data (digital data) with the frequency selected by the tuner 11 is supplied to the digital VTR 20 via the IEEE1394 interface 13. Although the IEEE1394 interface is a digital interface with a unique protocol, the IEEE1394 interface 13 on the set-top box 10 side and the IEEE1394 interface 21 on the digital VTR 20 side have input and output terminals ensuring transfer of the data between them without change. The digital data output from the IEEE1394 interface 21 of the digital VTR 20 is recorded on the tape 23 by the recorder 22 without any change.
Playback of the tape 23 by the digital VTR 20 will reproduce the original carrier data (digital data) at the same frequency. The reproduced digital data is supplied to the set-top box 10 via the IEEE1394 interfaces 21 and 13 so that the MPEG-2 decoder 12 decodes the digital data to restore the programs consisting of video, audio, data and so forth, and to output them from the set-top box 10.
With the foregoing configuration, the conventional digital data recording and reproducing system has the following problems. First, since the carrier data with the frequency selected by the tuner 11 is recorded straightforward, the plurality of programs are recorded as they are. Thus, the program data a user of the system does not actually desire are also recorded, which impairs the recording efficiency of the tape 23, and hence the recording duration of the tape 23.
A second problem relates to the discontinuity of the data input to the MPEG-2 decoder 12 of the set-top box 10. The recorded and reproduced digital data consist of not only data continuously received by the set-top box 10, but also discontinuous data, which is analogous to a single tape of an analog VTR including a plurality of recorded programs.
In the MPEG-2 decoding by the MPEG-2 decoder 12, since the input signal includes a plurality of multiplexed programs, the input signal is divided into specific programs and media according to the ISO/IEC 13818-1 standard. In the course of this, when the input signal contains discontinuity, the parameters for extracting the programs and media from the input signal can take unexpected values. Whether the abnormal parameters are considered a data error or not depends on the contents of the abnormality, or on the configuration for isolating the programs and media. In addition, since the detection of the parameters takes a substantial time period, the video, audio, data and so forth can be output with some errors until the abnormality is detected.
In addition, the MPEG-2 decoder 12 carries out video decoding according to ISO/IEC 13818-2 standard after separating the programs and media: It carries out such processings as variable length decoding, dequantization, inverse DCT (Discrete Cosine Transform) and motion compensated prediction. In the video decoding also, the input signal, that is, the signal passing through the isolation of the programs and media becomes discontinuous. Thus, it will cause some error in the decoding, thereby outputting abnormal video until the error of the input signal is detected and then the output of the video is halted. As in the isolation of the programs and media, it depends on the contents of the abnormality and the configuration of the processings whether the data is considered to be abnormal or not, and in addition, the error detection takes a non-negligible time period.
A common set-top box not involved in recording and reproduction can recognize timing its user switches the programs from his or her operation of a keyboard, for example. Thus, it can recognize the timing of the discontinuity of the data. Every time the discontinuity of the data occurs, the set-top box can restart the MPEG-2 decoder 12 in order to handle the input signal as new data. Thus, it can prevent abnormal video or audio decoded data from being output by starting receiving earlier than when carrying out error detection, and by halting the output of the video, audio and data (by displaying a particular color all over the screen in the case of video, for example).
However, as to the system required to carry out recording and reproduction, since the MPEG-2 decoder 12 cannot detect the discontinuity of the reproduced digital signal quickly, it has a problem of outputting abnormal decoded data for a long time.